When you hire an Artist, Band or a Performer, have agreed to a price and they turn up on time, do their job and act as a professional supplier of service, what they earn belongs to them. Not you.
Paying what you have agreed to is not 'optional'; it's their money. They have earned it.
Holding out the promise of future work, or future benefit, is not material. It's their money, in your pocket.
Reach in, take it out and hand it to them. It's simple.
If you can't, talk to them about it. Work out a deal like you would with any other supplier.
Treat a Band, Performer or Artist with the same professional courtesy you would provide to a plumber, electrician or your own staff. Bands, Artists & Performers are not second class citizens.
Artists, assert your rights.