Sheikh `Abd Allah al-`Umrânî, professor at al-Imâm University in Riyadh
The invalid condition will not affect the validity of the contract. Such a condition, however, would be deemed null and void in Islamic Law while the contract itself would still stand.
This ruling is drawn from Burayrah’s story in the hadîth related by `A'ishah.
If you know or have reason to believe that you will not be able to pay on time, then it will be unlawful to you to make a purchace in this manner. In that case, you are executing a contract of usury which is one of the gravest of sins.
However, if you believe that you can handle the payment from your own sources or from a friend in case of need, then it is lawful for you regardless of the invalid conditions.
If you are not sure, it is preferable to avoid getting involved in such a transaction to save yourself from violating Allah’s law.
Source: Islam Today