You can cancel the sale of the loan part by clicking the tab My Loans then Selling Loanparts tab.
Then click the downward arrow in the 'Asset Details' column, which will drop down more information.
Then on the right-hand side of the screen, you will see in the last column of the table an option saying 'Cancel Sale'. Click there to cancel the sale of your loan part.
*Please note if a loan part is offered for sale and then the sale of the loan part is cancelled, lenders will only earn interest again from the point they cancel the sale.