I am aplying for the K1 Visa, I'm filing I-134 for the Madrid embassy. My fiancee hasn't worked int he US for more than 2 years

Her dad will sign I-134 as a co-sponsor only if he can see an official document showing what is his real responsibility and when his liability will disappear. His dad stated very firmly that he will never sign I-864, in any way. My fiance, after working for some months, she will be my only Sponsor, and later myself with EAD and a job. So basically.... we depend on an official document that we can't find, where he can see clearly his responsibilities after signing I-134 and how long they will remain.