A player may be given an extension that is less than their current salary. However, such an extension has three restrictions.
1) The eligibility period for granting a player an extension below their current salary is only in the last year of their contract.
2) On top of the minimum and maximum years for contracts, the longest this type of extension can be is two years.
3) New salary may be no less than 70% of previous salary. Therefore, a player with market value of $2.5m, current one-year deal of $5m, can have an extension for two years equal to $3.5m (0.7 * 5 = 3.5).
So for Huston Street, he needs to be signed for 2.5*0.7 = 2M per year. Please let me know if you still want to sign him for one year.