239. Rent must be certain.

The rent must be certain, or must be so expressed that it can later be ascertained with certainty1. For this purpose, it is sufficient if, by calculation and upon the occurrence of certain events, the rent becomes certain; and, provided that it can be so ascertained from time to time, it is no objection that the rent is of fluctuating amount2.

An option to renew at such a rental as may be agreed is void unless means are provided for fixing the rent in default of agreement3; but, if the criteria for assessing the rent are stipulated but the machinery