A computer-implemented method for communicating data between computer systems in the functions of a source computer system and of a target computer system is performed by a data modification engine in a gateway computer that is communicatively connected between the computer systems. The engine receives an original data package from the source computer system with elements that are arranged according to a pre-defined data-structure. The engine accesses a rule template that corresponds to the data-structure and that provides a plurality of pre-defined rules. The engine separates the original data package into elements, processes the elements by digitally transforming them according to the rules, combines them to a modified data package and sends the modified data package to the target computer system.