Relationship between fields

I would like to create a relationship between two fields, e.g. Date field and Value field.
What I am trying to achieve is simplified sales reporting.
Each project has specific payment schedule, e.g. 01 Jan 2019: ?5,000, 01 Feb 2019: ?2,500 etc. I would like to be able to specify this payment schedule by manually selecting the date in one field and specifying the amount in a second field. 
Then I would like to be able to run a report listing all project revenue by month and by project (like cashflow).
Any help with this will be greatly appreciated.