Sublet changeover

So my subletter moved across the hall into his own office late last month, and the guy through the wall from me took over his space. In my mind, my first subletter was very fortunate that his space was taken over with no effort from me.  I released him from the next month's rent -- which I was not contractually obligated to do -- but then he wanted me to prorate and get rent from the other guy for the days that I let the other guy took over his space.  He calculated the amount that he should be paid as $53.16.

I laughed at him, told him that he was lucky that I wasn't enforcing the one month's notice thing, and told him that if he wanted the $53.16 he could ask my new subletter for it.  Well today me and the other guy finally executed a a sublease and I got a deposit and a check for April's rent, and I couldn't help but notice that he had also cut a check for $53.16.  Hmmm, I wonder who that could be for?