Load the games main library into IDA and find what function is used whenever the game wants to subtract in-game currency. When a game wants to subtract it would use a SUB instruction. When you find the correct function look for a SUB instruction and then hex edit it to an ADD. For example, if the instruction was SUB R0, R1 you would change it to ADD R0, R1. I use
ARM To HEX Converter Online to convert arm instructions to hex. If/When you convert the instruction look under 'Thumb HEX' for the hex you want ('ARM Hex' if the game is Unity/IL2CPP).