If your preferences are set such that transactions must be reversed, temporarily untick this under Setup, Preferences, Security. Underneath this setting, temporarily untick the Lock Period option as well.
Use the Transaction Journal to locate the cheque you want to void. Tick the arrow to the left of the transaction to open it. Change the cheque amount to $0 and click OK. This will change this to a Void (or $0) cheque.
Change the above preferences back to how they were.
The version won't matter, there are however a number of factors that could be in the way. You will need to provide more information, for example what type of payment are we referring to, Spend Money, Pay Bills, Paycheque, if yes, were timesheets used etc. What is you current financial year, what year were the cheques in.
When you say MYOB won't let you change the amount, is the arrow in Transaction journal greyed/hashed? When the transaction is opened, under Edit menu do you get a delete option or a reverse option, do you get an error message, if yes, what does it say.
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