Long time lurker here as I've never felt the need to post before!
A very well known singer announced a gig in my local area at the start of the year - I was delighted as my DH and I have loved his music since before he was this famous, so I was determined to try and get some tickets for us to see him. My DSis sent me a message to say that she and my DN wanted to go too, so could they come with us if I got their tickets with ours? No problem I thought, the more the merrier.
Ticket release day rolls around and I am lucky enough to get 4 tickets as they sold within minutes. From social media, I can see lots of people complaining that they couldn't get any, including some of my friends and DH's family too. I message my DSis to let her know I got her 2 tickets and she replies with 'Great! How much do I owe you?' so I reply with the cost and she says she will get the money to me.
This was 3 months ago now, the gig is a month away and you guessed it...I haven't seen the money! I've had 2 or 3 messages asking me to remind her of the amount, my bank details so she can transfer it...but still it's not in my bank account.
The amount is small enough for me to swallow if necessary but what is irking me is the principle of the matter! If I ask someone to get something for me, I pay them back immediately as I wouldn't want them to be out of pocket on my account. She and I are far from what you would call 'close' but she is a very selfish person and always has been. I know for a fact that if this were me owing her money, she would expect it to be paid back immediately. She is the sort of person who when she sells anything secondhand, expects the amount she paid for it brand new! She is a single parent with a teenage DD but she works full time and earns decent money, so she isn't hard up. She bought a brand new car at the start of the year and has been busy decorating and buying new furniture recently, so she cannot use the excuse that she hasn't got the money to give! I just suspect that because it's me, she thinks I can wait for it at her leisure and in the meantime can spend her money on other things 
So AIBU to tell her that if she doesn't pay the money by the end of the month at the very latest, that I will sell her 2 tickets to my friends who couldn't get them?