Is it permissible to give testimony that would harm the one who rented out the shop and cause him to lose his right to the commercial property, in exchange for renting the shop at a lower price?

If the lease contract violates the Sharia conditions due to an unspecified duration, then the contract must be annulled, and the tenant is not permitted to remain. The owner then has the choice to re-rent. If the contract is valid and for a specified duration, the first tenant may assign his right in exchange for an additional sum (key money), provided that the second tenant uses the premises in a way that causes no harm. This is what was affirmed by the Islamic Fiqh Academy. As for the name or trademark, it is a financial right that may be disposed of through sale or lease. Accordingly, if the first tenant's lease is valid and his testimony would harm him unjustly, then it is not permissible to give such testimony, based on the saying of the Prophet (peace be upon him): "There should be no harm nor reciprocating harm." However, if the testimony restores a right to its owner, then you must give it.