Bitcoin Payment Service provider

Hey,
i am searching for an BTC PSP where i can get an guaranteed exchange rate for my sell.

I will show the customer at the checkout page the price in Euro and BTC. If the price is for example €100) i want to calculate the Rate n BTC. This is for example in this second BTC 0,25.

The customer did pay the BTC and i received this BTC directly. Or the Payment provider.

My problem is that our accounting department don't want BTC. They need Euro. And if i change BTC in Euro a few days later i have a problem with the rates and the calculation of my products.

I am searching for an PSP which gan give me a guaranteed price for the BTC that i receive Euro.
It must be the same euro price which i displayed to the customer.

I did check Bitpay, bips and Gocoin. i think they made the settlement in BTC and the change in a exchange or from the PSP is on my risk a few days later.

Any ideas?

best regards
Onleines