Since toybox df lacks --output, use its POSIX mode instead, which is stable to parse:

# Free space on /data in KiB
df -Pk /data | awk 'NR==2 {print $4}'

-P guarantees the classic single-line format (no wrapped device names) and -k fixes blocks to 1 KiB, so columns are always: Filesystem 1024-blocks Used Available Capacity Mounted_on.

If you want bytes instead, stat -f works on Termux's toybox:

stat -f -c '%S * %f' /data | bc

• %S = fundamental block size
• %f = free blocks

Or pure-shell:

read size free < <(stat -f -c '%S %f' /data)
echo $((size * free))

Both avoid parsing human-readable -h output entirely.