Should I write the hours off to keep the peace?

Not as the first move. Writing off before showing anything tells the client the hours were soft, and it invites the same conversation next month. Show the work first. If you then decide to give something back, do it as a deliberate gesture rather than a retreat.

How detailed should the breakdown be?

Day level, with outcomes rather than activities. Detail below that tends to make things worse: it reads as supervision, it opens more lines to argue about, and it takes hours you cannot bill.

What if they dispute the invoice a second time?

Treat it as a scope conversation, not a billing one. Repeat disputes usually mean the work grew past what was agreed and nobody re-baselined. Reset the plan forward rather than relitigating the past invoice, and be honest with yourself about whether the account is still worth it.

Who usually ends up paying for the extra hours?

When the overrun came from scope creep rather than padding, buyers frequently absorb it, then correct the plan going forward. That is worth knowing before you concede: the useful outcome is a re-baselined scope, not a refund.

What to send when a client disputes an invoice

Before you reply, find the trigger

Disputes are rarely about arithmetic. They follow a bad surprise, a new finance process, a change of stakeholder, or a growing suspicion that the work should be faster than it is. If you answer the arithmetic and miss the trigger, you win the invoice and lose the account.

So the first message asks one question and offers the detail in the same breath. It costs you nothing and it changes what you are negotiating about.

The reply, in four moves

Acknowledge without defensiveness. A client asking is not an accusation, and treating it as one guarantees it becomes one.
Ask what prompted it. You need to know whether this is budget, a bad surprise, or a doubt about value.
Show, at day level, with outcomes attached. Not tasks, not minutes. What came out of each day.
Propose forward. Offer a call, and offer a way for them to see the work as it happens so the question does not return.

Copy this

Hi [name], Thanks for flagging it, and no problem at all asking. Before I send the detail, can you tell me what prompted it? If something came in higher than you expected, or a piece of work felt slower than it should have, I would rather fix that than just justify the number. In the meantime, here is what the hours covered, by day, with what came out of each one: [date] - [what shipped or was decided] [date] - [what shipped or was decided] [date] - [what shipped or was decided] Happy to walk through any of it on a call. [your name]

What belongs in the breakdown

Dates, and a plain sentence per day naming the outcome. "Reworked the Q3 plan after your Tuesday call" beats "strategy, 3 hours" every time.
Anything the client witnessed. Calls they attended, releases they saw, decisions they made.
A note on anything that ran long and why, before they find it. Overruns you surface read as candour, overruns they discover read as padding.

Leave out minute level detail, internal process, and any hint that you are keeping score. The goal is a document they can read in ninety seconds and recognise.

If they push back again

At that point the argument is about scope, not hours. Re-baseline the plan going forward and agree what the next month contains. If the same conversation happens a third time, the relationship has a trust problem that no record fixes, and suppliers who have been there consistently say the same thing: it is cheaper to leave than to keep proving yourself.

Making the next invoice uneventful

The suppliers who never receive this email are not the ones with better timesheets. They are the ones whose clients have already seen the work, week by week, before the invoice arrives. The companion guide covers how to build that habit, and this one covers what makes a record credible when a client is genuinely skeptical.