Can someone benefit me from getting a big customer of an item that I previously bought? How can I know the price of the item in the previous bill, especially if I already bought the item for a reduced price?
Your question is not clear.
Are you asking how to look up a prior sales transaction? You can Search for the customer’s name in the Sales Invoices tab. If the sale was by receipt instead of sales invoice, Search in the Receipts & Payments tab. The price will be listed on the transaction. Read the Guide: Search for records | Manager.
The price you paid for the item does not matter.
As @Tut describes, the price you actually paid should be recorded in Manager via the appropriate invoice or payment.
If you want Manager to tell you what discount you received last time, you will need to have recorded that information in Manager. That can be conveniently done be attaching the suppliers invoice (which typically lists that information) or you could enter it into the invoice / payment you create when entering each transaction into Manager.