Spurs should be fine next year - that is the great part of this deal being short term 1+1 deal. It's Pau's option next year, however, if he plays awful and it's not a good fit, he opts out. If he plays amazing and wants more money, he opts out. Only way it's an issue seemingly is if Pau gets injured badly and doesn't feel he can get a better deal.
Plus with everyone next year coming off the books, even having Pau at 15M gives the Spurs likely 28-30M in cap space (if they renounce the cap holds for players who's contracts are up).
Next year if Pau stays, SA would have:
LMA: 21.5M
Kawhi: 18.8M
Pau: 15M
TP: 15M
DG: 10M
Kyle: 2M
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82M in salaries with a projected cap of 107-110M.
Obviously this changes with whom SA signs this year and for how long, but that is the general math as we know it today.