Whenever you Validate an Invoice, It generates Accounting Entries to the relevant Journals (or Ledgers). So You need to rollback them first and then use it in the Contract. You can create a function for reverse the effects of Validating Invoice and bring the invoice back in draft state and then add it to the Contract.

No, you can not add already validated invoice to your contract, but you can add draft invoice to your contract by changing "Source Document" (by default field invisible, auto filled)... 

Thanks.