dd allows you to specify your seek, skip and count values as bytes instead of blocks using the iflag/oflag options seek_bytes, skip_bytes and count_bytes.
So to read the first MB of data 100GB into a file you can:
> dd if=/tmp/file1 bs=2M skip=100G count=1M iflag=count_bytes,skip_bytes
It is also in Coreutils 8.22 in RHEL7.
Edit: https://man7.org/linux/man-pages/man1/dd.1.html
Edit 2 : I just found this in the coreutils 9.0 changelog:
dd now counts bytes instead of blocks if a block count ends in "B".
For example, 'dd count=100KiB' now copies 100 KiB of data, not
102,400 blocks of data. The flags count_bytes, skip_bytes and
seek_bytes are therefore obsolescent and are no longer documented,
though they still work.