Is it lawful to sell checks and bills,
even for a lesser price?
Selling checks in the mentioned way is impermissible, as it is considered Riba Al-Nasi'ah (usury of delay, conditional excess for delay of payment) and Riba Al-Fadl (usury of excess, selling an item for another of the same type, on the spot, but in excess).
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.