Upon the expiration of said nine (9) months from and after the completion and acceptance of the work, the City shall pay to the Contractor the said five (5) per cent hereby retained, less such sums as may have been retained hereunder. 21. Unpaid Claims against Contractor. The Contractor shall furnish the City with satisfactory evidence that all persons who have done work or furnished materials under the contract, and have given written notices to the City, before and within ten (10) days after the final completion and acceptance of the whole work under the contract, that any balance for such work or materials is due and unpaid, have been fully paid or satisfactorily secured. And in case such evidence is not furnished as aforesaid, such amount as may be necessary to meet the claims of the persons aforesaid shall be fully discharged or such notices withdrawn. 22. Delays and Difficulties. The Contractor shall not be entitled to any claims for damages on account of postponement or delay in the work occasioned by forces beyond the control of the City, nor for postponement or delay in the work where ten (10) days written notice has been given the Contractor of such postponement or delay, nor where unforeseen difficulties are encountered in the prosecution of the work. In the event of a postponement or delay ordered in writing by the City the time of completion of the contract shall be extended a number of days equal to the number of days that the work has been postponed or delayed. 23. Assignment of Contract. The Contractor shall not assign by power of attorney or otherwise, nor sublet the work or any part thereof, without the previous written consent of the party of the first part, and shall not either legally nor equitably assign any of the moneys payable under this agreement or his claim thereto unless by and with the consent of the party of the first part. 24. Cleaning Up. On or before the completion of the work, the Contractor shall, without charge therefor, tear down and remove all buildings and other structures built by him, shall remove all rubbish of all kinds from any grounds which he has occupied, and shall leave the line of the work in a clean and neat condition.