Generally spoken it is not possible. The transaction number for itself is not accessible at the moment the donor recall the paying slip. The reason for this can be seen in the fact, that the transaction does not exist at that particular moment.
In detail, after the donor recalls the paying-in slips he does a payment promise and no real payment. It is possible, that the donor fills in the paying-in slip or changes to another way of payment such as online banking.
Nevertheless, there are two other options how to assign donations to paying-in slips:
1. Use an extra bank account just for paying-in slips
One possibility in order to retrace donations via paying-in slips is the usage of an extra account for paying-in slips. This option needs no further support of us.
2. Use Merchant Transaction Keys
Another, more elegant way is the usage of a Merchant Transaction Keys. With this solution, you won't need an extra bank account.
While triggering a donation via our RaiseNow widget, there will be generated a key, send to your widget and further to our payment API. This key is called Merchant Transaction key.
For this option, our technical support would love to get in contact with your web agency. Send us the contact details of your agency to firstname.lastname@example.org. We will send you an offer for the usage of Merchant Transactions Keys.