manual rebate--paid amount field became gray

Dear SAP Gurus,

I have a problame in my production system. the issue is we are paying rebate for last quarter. the maxmum amount fields amount is .7million but the paid amount fields becomes gray. my rebate validity period is 01.01.2012 to 31.12.2012. please provide some helping clue. your advice highly appericiable.

thanking you in advance.