If you want to buy something that costs more than your available balance, you should not use your Benepass card. If you try to buy something that costs more than the balance you have on your account, the card will decline. If you have multiple benefits with a total balance high enough to make the purchase, that still won't help: Benepass can only charge a single benefit at a time.
If your employer offers reimbursements through Benepass, you can make the purchase on your personal card and reimburse it in installments. For more information about how to submit an expense for reimbursement, check out this article.
If your employer doesn't offer reimbursements through Benepass, the best course of action is to ask the merchant whether they can split the payment between two cards (your Benepass card and a personal card), or whether a partial payment plan is possible. Please keep in mind that your Benepass card is a credit card, and will only work on platforms where credit cards are accepted.