programming a old omron rs81 electric cash register?

we lost the code for the payment key so that if you hit the digital numbers1-9 an amount like $100 for payment and you want it to leave you with a credit balance if you hit the cash key the machine does not do that any more as I mistakingly hit the wrong code when i was changing the tax rate by programming through the p2 key mode for programming the machine. It can do a change function which we do not need but we can not get it to give us a credit balance. we need the exact steps to program the correct code