Triggered by https://github.com/mimblewimble/grin-pm/pull/278.
When I raised my funding request payment on April 1, the GBP/BTC exchange rate meant the withdrawal would be 1.50 BTC.
However, it took until the next day, April 2, for the mu-sig transaction to be signed and broadcasted. At the closing time that day, as per /pull/278, the price had moved so that 1.50 BTC equaled USD 10,190.43.
Based on the prevailing exchange rates on April 2, this then implies that I was paid £723.39 too much in this funding request.
Looking back over my historical funding requests as per the spending log:
- April 12: USD 9,000, paid exact, +/- 0
- October 11: USD 7330.97, i.e. USD 169.03 less than entitled, translating to £133.62 too little. 
- Jan 15: USD 9719.62, translating to £47 too little. 
So, in net, I owe the fund:
£723.39 - £133.62 - £47 = £542.77
I propose I deduct that from any future funding request, and if there is no future funding request approved, that I pay back the equivalent according to the prevailing BTC rates at the time.
A way to reduce these volatile swings in exchange rates is to tighten the time from the withdrawal request to when the transaction is actually broadcast. Better co-ordination during signing helps, and something towards this end I think makes sense to be added to the spending guidelines rfc that’s currently in FCP.
 Source: https://i.ibb.co/fHWW0sX/Screenshot-2020-04-01-at-11-50-57.png
(10190.43/1.2392) - 7500 = 723.39
169.03*0.7905 = 133.62
(9719.62*0.7668) - 7500 = -47.00