The property you have received from your customer is fully taxable and should be included in your tax return. I have done some research on the matter and have found a case that has similar facts to your situation. In Commissioner v. Duberstein, two individuals, who were both presidents of their respective companies, talked regularly to transact their businesses with each other. Bernan, the president of Mohawk, asked Duberstein, the president of Duberstein Iron and Metal Company, if he knew of potential customers for Mohawk. Duberstein said yes and provided Bernan with the names of potential customers. One day, Bernan phoned Duberstein and said that the information he gave was helpful and that he wanted to give Duberstein a president. Duberstein ended up accepting the present, which was a Cadillac. Mohawk deducted the value of the Cadillac as a business expense and Dubersten did not include the value of the Cadillac in gross income for 1951, deeming it as a gift.
The Supreme Court used different tests in order to help classify this transaction as a gift or a non-gift exchange. One test provided that, “if the payment proceeds primarily from the ‘constraining force of any moral or legal duty,’ or from the ‘incentive of anticipated benefit’ of an economic nature, it is not a gift. And, conversely, ‘where the payment is in return for services rendered, it is irrelevant that the donor derives no economic benefit from it.’” This outlines one true definition of an exchange of property from a business to an individual, which differs greatly from the definition of a gift. The definition of a gift in the statutory sense, proceeds from a “detached and disinterested generosity” and is made “out of affection, respect, admiration, charity or like impulses”. The Government proposed a second test about gifts. The propositions of the second test include that “payments by an employer to an employee, even though voluntary, ought, by and large, to be taxable; that the concept of a gift is inconsistent with a payment being a deductible business expense; that a gift involves “personal” elements; that a business corporation cannot properly make a gift of its assets.”
The Mercedes-Benz you received from your customer would be viewed by the court as a recompense for your services, or an inducement for you to be of further service in the future. Also, the car was given to you in return for services rendered, which immediately fails the first test given by the Supreme Court.