[1] It is established that a contemporaneous parol agreement is not admissible in evidence to show the intention of the parties was other than that clearly expressed in the writing signed by them and evidencing their agreement. Formby v. Williams, 203 Ala. 14, 19, 81 South. 682, and authorities there collected; Planters’ Chem. & Oil Co. v. Steames, 189 Ala. 503, 66 South. 699; Morris v. Robbinson, 80 Ala. 291. The paper writing in question is unambiguous, and no error was committed in rejecting evidence of the contemporaneous parol agreement made at the time the mortgage was executed, and not carried therein, that it was to draw interest from the date of its execution, and not from the date of maturity as therein written. The mortgage and note, *481 contrary to the parol agreement, speak for themselves and may not be varied by parol.

[2, 3] In order to prevent a party to a contract from recovering for overpayment of moneys, he must have overpaid with full knowledge of all the material facts. Morrill v. Brantley, 133 Ala. 537, 538, 31 South. 847; Rutherford v. McIvor, 21 Ala. 750, 756; 30 Cyc. 1300. In Merrill v. Brantley, Mr. Justice Sharpe said:

“To disentitle a party, on such ground, to recover [money paid without consideration], it must appear that he had actual knowledge of the attendant facts which were calculated to influence the making or withholding of the payment.”

See Hinds v. Wiles, 12 Ala. App. 596, 68 South. 556; Rutherford v. McIvor, supra; Youngblood v. Youngblood, 54 Ala. 486; Young v. Lehman, Durr & Co., 63 Ala. 519.

The general text of 30 Cyc. 1319, 1320, supports the proposition that an overpayment of interest by mistake can be recovered. The fact that a person, when making a payment, had the means of knowing the facts, does not of itself ordinarily preclude him from recovering the money, if he did not have actual knowledge that the sum paid was an overpayment. Merrill v. Brantley, supra; Rutherford v. McIvor, supra: Russell v. Richard, 6 Ala. App. 73; 1 Hinds v. Wiles, supra; Wilson v. Sergeant, 12 Ala. 778; 30 Cyc. 1320. where the authorities of American and English courts are collected. The recovery is rested on the fact that one has money which ex requo et bono belongs to another. Jasper Co. v. K. C., M. & B. R. R. Co., 90 Ala. 416, 14 South. 546, 42 Am. St. Rep. 75: Christie v. Dyer, 205 Ala. 572, 88 South. 668; Christie v. Durden, 205 Ala. 571, 88 South. 667. If defendant knew that the paper was written as it was, for 81,000 due January 1, 1919, but supposed that its proper construction imposed upon him the payment of interest from date of execution, and by reason of this construction made the overpayment, he would not be permitted to recover the overpayment, since it was paid under a mistake of law, and' not by reason of a mistake of fact. Young v. Lehman, Durr & Co., supra; Town Council of Cahaba v. Burnett, 34 Ala. 400. A mistake of fact may he said to exist when some existing fact is unknown or a supposed fact does not exist; and this would apply as to the terms of the instrument or calculations of interest made thereon.

[4] There was no motion for a new trial in the court below. The issue was fully and fairly submitted to the jury, and judgment was for plaintiff. The burden of proof was on defendant under his plea of recoupment (Lysle Mill. Co. v. Nor. Ala. Gro. Co., 201 Ala. 222, 77 South. 748) to establish his counterclaim by reason of overpayment of interest by mistake of fact. Whether there was a mistake of fact was an issue for the jury (Tuscaloosa Co. v. Roster, 132 Ala. 392, 31 South. 587; Ewing v. Peck, 26 Ala. 413; 30 Cyc. 1325); and we find no reversible error after inspection of the whole record.

The judgment of the circuit court is affirmed.

Affirmed.