She can obligate herself:
a- It is possible for her to have assets that her husband does not have rights over (if someone gives her something ‘al manas sh’en l’baalech reshus baah). In addition, her own personal possessions belong to her.
b- the obligation exists; should her husband die/divorce, the obligation would be applicable.
She is not considered incompetent. In addition, small sums that she may use without her husband permission, in that realm she may make agreements ( many realms of the home are purely her’s; in these areas, her husband clearly acqueises to whatever her will is).