My own two solutions are:
1) Leave 4$ and write on the receipt: "Pi$ for the mail and (5-Pi)$ for the tip."
2) Pay integrally in a foreign currency while waiting for the exchange rate to cut through Pi$.
(#2 has met with criticism in that the exchange rate can never be known in advance).
But, another solver came up with this: On the island of Yap, the currency consists of stones, and a large crafted stone is given the value ascribed to it by the stonemason. So, one might politely write to a Yap craftsman asking if a "Pi stone" could kindly be carved.
To which I added that similarly an "i=sqrt(-1)" stone might be cut, so as to introduce imaginary currency into our financial markets. To which others replied that this had already been done long ago.
There is a lot more available here:
https://www.quora.com/I-walk-into-a-restaurant-for-lunch-and-my-check-comes-to-%CF%80-dollars-The-restaurant-will-only-accept-cash-for-the-exact-amount-nothing-more-nothing-less-What-are-my-options
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