I have occasionally had to have a sub for me in the swing band.
There were two separate occasions where at the last minute I was suddenly available for the gig.
Being as the band will always be better with the regular players, both times I called the sub and told him I was available for the gig, thanked him for being available and paid him for the gig.
I played for nothing.
If you were able and available for the gig, you should have been paid also.