Cost Loading Issues Pls Pls Help

Hi I am at my wits end with attempting to cost load activity bars correctly I have tried everything.

I am simply wanting to assign an amount of money to an activity, then when this activity is updated for it to update the spend & remaining budget accordingly.

e.g. if I have an activity which is 10 days long and has £10k assigned to it, if this activity were 3 days (30%) complete I want it to then show £3k spent with £7k remaining. I have created a resource called £ and can get it to show the original amount but as soon as I move the start date, or update the durations, % complete etc it amends the figures & gives me random figures linked to the units per day etc etc

Can anyone help as at the moment I am manually working the figures out & each time I move the data date recalculating the spend & remaining!!

Thanks