What if the price changes between the check and the payment?

Why it works this way

Tanilo checks the inputs you send: the offer, and the pricing terms or cap you supply. Tanilo does not execute, block or authorize your payment. It doesn't hold the seller's price or watch for changes afterwards. The result describes those inputs, and a signed receipt is issued when signing succeeds.

The pattern we recommend

Check the offer before committing.
Just before paying, compare the offer you're about to pay with the offer that was checked. Compare every field of the offer: price, quantity, currency, seller, product identifier, and any unit or offer time.
If the offer is unchanged and the terms or cap you supplied still apply, act on the result under your own policy. If anything needs updating, check again with the updated inputs and apply your policy to the new result.
Keep the complete input and the receipt, and keep access to the rule version the receipt names, so you can replay the check later.

Step 2 is yours. Tanilo records the inputs and the result; your workflow compares the purchase with them and decides whether to go ahead.

Checking and paying are still two separate steps, even with this comparison. Your workflow has to handle anything that changes in between.

What the earlier receipt still means

It's still the record of the result for the earlier inputs. A later price change doesn't alter that record or its result. It just isn't about the new offer.

Spend caps: two purchases at once

The spend cap check compares the offer's total (unit price × quantity) with the cap you supply: either the per-purchase cap, or the period cap minus the already-spent amount you supply. It keeps no running total. If two purchases are checked against the same remaining cap at the same time, both can pass even though together they exceed it. Tracking spending and coordinating purchases made at the same time is your system's job, and recording spending only after payment doesn't prevent this. The check also doesn't establish that the cap applies to this purchase, or that the already-spent amount you supply is complete or current.