Keep your own record of every payment

Keep one sheet that never lies to you: every payment request as it arrived, what was actually paid, when, and what the running total says against the contract price. It takes minutes a month, and it is the difference between knowing and hoping when the account gets discussed at the end. Builds where nobody kept this sheet end with the final bill negotiated against memory - and memory, a year of Fridays later, always favours whoever wrote things down. Bank statements can reconstruct the history if it comes to that, but a record kept as you go costs nothing and argues better.