Tex. Tax Code § 151.055

(a) If a person purchases tangible personal property by means of a sale for resale for the purpose of renting or leasing the property for use but subsequently sells the property in an occasional sale before the person has collected and paid to the state an amount of sales tax on rental or lease charges equal to the amount of sales tax that would have been due if the person had not acquired the property at a sale for resale, the person at the time of the occasional sale shall include in his receipts from taxable sales the amount by which the purchase price of the item at the occasional sale exceeds the amount received from renting or leasing the property.

(b) If tangible personal property is rented or leased under an agreement that provides that all or a portion of the rental or lease payments may be credited against the purchase price of the item, the lessor shall collect the sales tax on the sales price, including the sum of all lease or rental payments for the term of the lease or rental, at the time the purchaser takes possession of the property or when the first payment is due, whichever period is the earlier. If the purchaser-lessee returns the taxable item to the seller-lessor before the end of the lease or rental period without having acquired title to the property, the seller-lessor may take a credit against other taxes due under this chapter or claim a refund as provided by this code for an amount equal to the amount of the taxes paid on the unpaid portion of the sales price.