Model transformations are the linking element between the different levels of abstraction in the model-based user interface development. They map source elements onto target elements and define rules for the execution of these mappings. Approaches for the reuse of transformation rules use formal transformation models, which only specify the mappings and abstract from the implementation. Current solutions are usually only able to describe unambiguous (1-on-1) mappings. In general, however, there are ambiguous (1-on-n) mappings from which the unambiguous mappings are only chosen during the design process. The knowledge which source element can be mapped onto which target elements is to date not being formalized. This paper therefore presents a proposal for an ambiguous mapping and transformation model and describes its usage in an iterative development process.