In any cause founded on contract, when any part of the principal or interest shall have been voluntarily paid, or an acknowledgment of an existing liability, debt or claim, or any promise to pay the same shall have been made in writing, an action may be brought in such case within the period prescribed for the same, after such payment, acknowledgment or promise; Provided, that the provisions of this section shall not be applicable to real estate mortgages which have become barred under the provisions of section 25-202 as against subsequent encumbrancers and purchasers for value.