I want to resign and i need some advice.?

I was hired by this small company to "clean up" the point of sale system ( Dac Easy) and to generate a profit and loss statement from said system, however due to a lot of tedious issues with the software that caused my system to crash the report couldn't be done. I mentioned this to the boss and he seemed to care less, scared to loss my job i create a spread sheet with all the appropriated formulas at home, took it to work and entered the info so i can get a report to hand in. Can i delete this spread sheet? Does it belong to me? This is additional work i did that i wasn't paid for.