SQL Server : database free space vs data file free space
There are apparently errors in the calculations in SSMS.

Shrink Database dialog

Currently allocated space: show the allocated space for both data and log. Correct.

Available free space: show free space for only data, doesn't include log. Incorrect.

The calculating of the free space is off. It divides the full database size (including log) with the free space for the data file (only). Also, it presents the full database size (including log) and for free space, it only show free space for data.

Shrink File dialog

Referring to data file. This is inconsistent. For one of my databases, is show the correct info for the primary data and only file. For a different database (stack overflow 10 GB), it is way off with free space (both number and percentage). For the SO db, it shows only about 600 MB used, when in fact it is about 1.6 GB. My guess is that there is an overflow in a variable in the host language within SSMS causing a wraparound (if it were in TSQL you'd get an error instead) 

I.e. disregard the GUI. Collect info from some more correct query/method (what Niels suggested for instance - I use my own sp_dbinfo for these things). and then use the DBCC SHRINK commands directly (preferably DBCC SHRINKFILE). 

And, of course don't shrink unless you really really really need that disk space, it is a significant amount of disk space and you are willing to pay the price for shrinking.