ULN2003A to sink current when collector volatge less than base voltage
the ULN2003A datasheet here only states the maximum voltage that can be sinked, which is 50v.


First, we don't talk about "sinking" voltage. We sink current. The spec you're referring to says that when the ULN2003A is in cut-off mode, it can withstand 50 V applied to the collector and not leak more than 100 uA.


  Thus what is the minimum collector voltage with respect to base which can be sinked?


Again, you can't "sink" voltage. The spec you are looking for is the VCE saturation voltage. It's included in the specs you posted, and the maximum is 1.1 V.


  I always thought that the base voltage should be greater than the collector voltage


In saturation, we usually estimate that a typical NPN BJT can pull its collector node down to about 0.2 V. Since this is a darlington configuration, the overall collector-emitter voltage drop for the ULN2003A includes the base-emitter voltage of the "lower" transistor, plus the \$V_{CE}\$ voltage of the "upper" transistor. Using the typical values this is 0.7 + 0.2 or 0.9 V, which is exactly the number given in the datasheet for the typical behavior at low currents.