Need formula to enter data every 28 days

Sounds simple but I cannot get it to work. Sheet layout is as follows

Column A date

Column B Rent

Column C Rent Benefit

Column D My Payment

Column B formula =IF(AND(WEEKDAY($A7,2)=1,A7<=$A$5),$B$5,0) enters amount every Monday

Column C formula =IF($A6=43839,$C$5,0) Formula for the first cell only

Column C formula =IF(AND(A7<=$A$5,A7=A$6+28),$C$5,0) enters amount in cell A6 then 28 days after

I cannot drag the bottom right hand corner of the cell and get the formula correct. So it should be A6+56 then A6+84 and so on.

Although I can manually do it I would like to know how to get the formula to work.

Easier if you look at the sheet