We had this last night, lovely small bar in Kent which was unexpectedly rammed for our gig, fixed fee agreed, owner comes up at the end of the gig to tell us she's adding some cash to the fee as we went down so well. Confused the singer who now couldn't do the maths for the split
Re the OP, it's frustrating but if the agreed performance fee was based on bar receipts - which we've had a few times - than it leaves it open, in short you have to really trust the place to go ahead and gig with that being the arrangement. If you agreed a fixed fee for the gig then yes, it's very poor and you should go medieval on their social media about it 👍