All deductions, credits, income, gains, losses, and distributions of a limited partnership shall be allocated among the partners, and among classes of partners, in the manner provided in writing in the partnership agreement. If the partnership agreement does not so provide in writing, all allocations shall be made on the basis of the amount, as stated in the partnership records required to be kept pursuant to Code Section 14-9-105, of the contributions made by each partner to the extent that they have been received by the partnership and have not been returned.