I would have given her the gift as promised.

 
I had something similar happen recently, one of my SB's came over not long ago, and she too had "just" started her period, so she offered me her ass instead. SOLD!!!  

 
Now if an SB shows up to a date and begs off from having sex completely because of an un-announced visit from Aunt Flo I might give her gas money, but I would feel under no obligation to give her our agreed PPM.

If you are seeing an SB on a regular or semi-regular basis, you will have dates when she's in going through her cycle.  Just like a civvy GF.  I've had this discussion any number of times with any number of SB's. Usually, she lets me know when we schedule a meet, and we can discuss implications/specific activities.   Some (a smaller %) of my SB's have simply asked to schedule a different day.  
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I remember a similar situation with a porn star SB. Not a period, but she had added a new tattoo next to her labia - I really have no idea why TF she would do that, but I am old and she ended up being Ultra-BSC, so... Anyway, of course she did not tell me about this until she got into my car to go back to my place. And also of course, she immediately showed me the new tatt by pulling up her dress (no panties) while we were in traffic.  And by pulling up her dress, I mean pulling it off over her head and sitting naked in my car. This caught the eye of other drivers, pedestrians and when she opened the window and started calling out to anyone in earshot. Like I said, Ultra-BSC.  
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Anyway, the tatt guy told her she could not have vaginal sex or booze for 48 hours. So we get to my place and she grabs a bottle of vodka and tell me to fuck her ass as hard as I could.  
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I did not adjust the allowance that day.  But I did "punish" her for denying my entrance to my favorite parking spot.  
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But ultimately, the BSC grew too tiresome, and I had to drop her.  Too bad, of course. The sex was off the chats.  
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What were we talking about? Oh yeah, periods. Don't sweat it, Dude. It's part of the process.
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Life is good.
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The Cat

I've had several dates scheduled well in advance only to be informed on the day or day before show time that her period had arrived.  I always ask if we can postpone and that request has always been agreeable.  I've also had a few experiences where we were already having intercourse and it felt unusually wet down there and it turned out she had just gotten her period while we were fully involved. Period sex is not my thing either so we stop and get cleaned up.The SB has always been very embarrassed but I take it in stride.  I always tell her no problem, I like dating Earth Girls and this is just what you get sometimes! And I always give the expected $ugar in those circumstances.  If a SB showed up as planned and only then informed me she was on her period I'd be annoyed and would not want to continue the date.  But I'm glad I've never had to send someone home empty handed in that situation.

In general, I take it in stride. In one case, I had an ATF, a provider, call me on her way to announce that her period had arrived. I told her not to bother coming, which I regret. I was young and dumb and it was rude and thoughtless.  She was understandably miffed and it was the beginning of the end of what had been a fun relationship.

Because each woman's rate of flow varies, whether or not to have intercourse is an individual decision to be arrived at together. Sometimes it's no big deal and under the right circumstances can be hot. But it does keep me from indulging in oral, one of my very favorite things.

My current regular gal is okay with pretty much anything, including anal. However, she told me that she doesn't like period sex and, because she isn't all that regular and doesn't get any physical sign in advance, it has been difficult to plan around her periods.  

I always assumed that she was uneasy with it because of the mess; she is Asian and always super clean and fresh. However, the last time we were together she told me that she didn't like period sex because her breasts get very sensitive at that time and she doesn't want them played with, licked, or sucked. Adding her period flow to our mix of fluids doesn't bother her in the least. We agreed that her period wouldn't stop us in the future, but that I would keep my hands, mouth, and everything else below her waist or above her shoulders.

When my SB’s have encountered Aunt Flo I’ve always been gracious.  I’m not a fan of period sex, with all that comes with it.  If SHE says she’d like to please me orally, then I’ll take her up on it.  When that happens I usually get 2 for the price of one.  If she isn’t in the mood because of her period that’s okay too.   I still give her the allowance knowing she needs it.  The rewards come back 10 times.