Service Contract

at the moment, I have a little problem (gonna be big in a next few weeks) with service contract. the problem is my customer create a contract which has amount 50,000.- per year, invoice period is "month" and Amount per period is "4,166.67.-" contract start from 25 Jan 05 - 24 Jan 06. navision will create invoices with amounth 4,166.67 for everymonth (exactly right) except the first invoice by cutting 25 Jan 05 - 31 Jan 05 to start next invoice at the begin of next month and the last invoice 01 Jan 06 - 24 Jan 06. for the first and the last invoice make contract amounth wrong. In fact, my customer should get 50,000 baht but navision create entire invoice 50,0097.97 (Approximately) which is wrong. What should I do to make navision create invoice as I want?