Is it permissible to change only the lenses of glasses for the price of a complete pair of glasses and submit an invoice showing it as a complete pair of glasses to obtain the full amount paid, because the amount allocated for lenses is insufficient?

Muslims are bound by their conditions, and contracts must be fulfilled, as Allah Almighty says: "O you who have believed, fulfill [all] contracts." Regarding the right granted by the company, if it is a fixed sum, you have the freedom to dispose of it, whether to change the entire glasses or just the lenses. However, if it is designated for a specific purpose (such as changing glasses only when needed), it is not permissible to dispose of it except for the designated purpose. It is also not permissible to forge a sales invoice claiming that you changed your glasses when you did not.