Locating empty field groups for saving data
Technically, there is no such thing as if-loops. Your code is just a bunch of if-else statements (sorry, this was the language police speaking)

What your code actually needs, is actually a real loop. Use an array or a list of a record, TLoanProduct and loop through it. Your current approach allows a maximum of six products in one loan, it involves a lot of code duplication which in itself is a code smell, and it could be a bit weird if the values for Product 3 were set but not Product 1.

My Delphi skills are a bit rusty, but consider changing TLoan into an object or a class instead of a pure record. That way you could have the Save method as part of the TLoan object/class.

This is some code for how I think it should look. It's been a long while since I worked with Delphi so this might not be 100% correct code, but it should get you started in the right direction.

TLoanProduct = record
    ProductID: integer;
    ProductTitle: string[80];
    ProductAuthor: string[80];
    Returned: boolean;
end;
TLoan = record
    LoanUniqueId: integer; { of loan record }
    DateDue: TDateTime; { date when products are due to be returned }
    MemberUniqueID: integer; { from MemberDetails File }
    MemberName: string[50]; { from MemberDetails File }
    MemberSecondName: string[50]; { from MemberDetails File }
    AmountBorrowed: integer; { amount of items currently on loan }
    LoanProducts: array [0..5] of TLoanProduct;
end;

...

procedure Save;
{ procedure to locate which field groups are empty so product and loan informati
 - on can be saved }
var i: integer;
begin
  for i := 0 to High(LoanRecords.LoanProducts) do
  if LoanRecords.LoanProducts[i].ProductID = '' then
  begin
    LoanRecords.LoanProducts[i].ProductID := ProductRecords.UniqueId;
    LoanRecords.LoanProducts[i].ProductTitle := ProductRecords.ProductTitle;
    LoanRecords.LoanProducts[i].ProductAuthor := ProductRecords.ProductAuthor;
    Exit; // Exit the method so that this is only applied for one element
  end;
end;