'Saldoziehung' refers to the practice of lumping sales and purchases with a particular trading partner together and settling net ('running a current account'?...not quite sure what we say).

This is rather complex legally, and one effect is that legal claims/rights are transferred from the individual items to an abstract claim on the net balance. This in turn is why standard AGBs insist on keeping retention of title for individual items (or all items taken together) despite 'Saldoziehung' or 'laufende Rechnung'.

Here, in keeping with that, they're saying that their retention of title on items not yet paid for will serve as security for the amount owing.